This invention relates to a pattern forming method for forming fine patterns with great throughput and a lithographic system for realizing the method.
Photolithography has been used during the manufacture of a wide variety of semiconductor devices because of its simplicity and low cost. Continually advancing technical innovation has brought about use of shorter wavelength in recent years (e.g., use of a KrF excimer laser light source), which has realized the manufacture of patterns 0.25 .mu.m or less in width. To miniaturize the devices further, a ArF excimer laser light source and a Levenson-type phase-shift mask both with much shorter wavelength are being developed. It is expected that they will be used as mass-production lithographic tools conforming to 0.15 .mu.m rules. There are a lot of problems to be solved before such tools are realized. This elongates the time required to develop them, leading to the fear that the development cannot catch up with the pace at which the devices are being miniaturized.
In the case of electron-beam lithography, a first candidate for post-photolithography, it has been proved that use of a finely focused beam produces patterns as fine as 0.01 .mu.m. Although there seems to be no problem for the time being from the viewpoint of miniaturization, electron-beam lithography has a throughput problem in terms of device mass-production tools. Because fine patterns are drawn one by one, the inevitable result is that the time needed to draw the patterns is long. To make the drawing time shorter, several systems have been developed. One of them is a cell projection system that draws patterns in such a manner that part of repetitive portions of the ULSI patterns are lumped together. Use of those systems, however, have not succeeded in achieving throughput coming up to the throughput of photolithography.
As a method of increasing the throughput of electron-beam lithography, the following method has been proposed: pattern transfer to the same resist is performed by light exposure and electron-beam exposure to reduce the area exposed to the electron-beam and increase the number of wafers that the electron-beam drawing machine can process in an hour. That is, a light beam and an electron beam are used to expose layer.
Jpn. Pat. Appln. KOKAI Publication No. 4-155812 has disclosed that in a pattern formation lithographic process, pattern transfer to the same resist is performed by exposure to a light beam and an electron beam with a phase shift mask. In the approach, most of the patterns forming the elements are transferred through a phase shift mask and the portions where defects have occurred because of the arrangement of the phase shifter are repaired with the electron beam. This minimizes the area drawn by the electron beam and increases the number of wafers that the electron-beam drawing machine can process in an hour.
With this method, although the drawing area need not be large, it is impossible to transfer patterns smaller than the critical resolution of the phase shift mask. Consequently, the method will not be able to deal with the miniaturization of future devices.
In manufacturing few-of-a-kind elements, it takes considerable time to produce a mask. To overcome this problem, Jpn. Pat. Appln. KOKAI Publication No. 1-293616 has disclosed the following method: a function block common to various semiconductor elements is printed on the same resist by light exposure and then a pattern peculiar to each semiconductor element is drawn using an electron beam. Namely, a mask for the portions common to the individual elements is made beforehand and only the remaining portions where the pattern differs from element to element are drawn using an electron beam. This method makes it possible to shorten the time needed to design the elements and manufacture them, because it is not necessary to produce a mask for each pattern.
The method, however, cannot deal with a case where the function block contains patterns smaller than the critical resolution of light exposure. In the method, most of the patterns to be drawn with an electron beam are interconnections. To form the interconnections by electron beam exposure, the patterns have to be drawn one by one. Inevitably, this process takes a lot of time.
As described above, the conventional method of projecting a light beam and an electron beam on the same layer to improve the throughput has the following problems: the resolving power of electron-beam exposure is not used sufficiently and the throughput is not raised to that of the optical stepper.